It has long been recognized in the field of glass-ceramics that, upon heat treating a precursor glass body to convert it into a glass-ceramic body, a relatively small, but nevertheless significant, shrinkage of the body customarily occurs due to densification taking place therein as crystals are developed and grown in situ. This phenomenon is illustrated in FIG. 1 which depicts a typical length-change curve with time and temperature exhibited by a precursor glass capable of being transformed via heat treatment into a glass-ceramic body. Thus, the AB portion of the curve reflects the thermal expansion manifested by the glass as it is heated to its transition temperature B. At that temperature (slightly above the annealing point of the glass) phase separation and/or nucleation is initiated in the glass. C designates the temperature at which crystal growth commences within the glass with consequent densification thereof. The segment CD represents the rapid shrinkage of the body as the temperature is raised to expedite crystallization therein, followed by a general leveling off as completion of the desired crystallization of the parent glass to a glass-ceramic body is accomplished. Typically, as is illustrated in FIG. 1, a crystallization hold or dwell period at a particular temperature is utilized to complete crystallization but that practice is not mandatory. All that is required is exposure to temperatures above C. It will be appreciated that with certain glass compositions a series of crystal phases may be developed as the temperature of the parent glass is raised. Accordingly, C represents the temperature at which growth of the desired crystal phase commences. DE indicates the thermal contraction of the glass-ceramic as it is cooled to room temperature. The obvious decrease in size experienced by the precursor glass body is an inherent concomitant of the densification occurring during the crystallization in situ thereof.
In most commercially-marketed glass-ceramic products, e.g., culinary ware, dinnerware, radomes, etc., the overall dimensional specifications are not so stringent but what a modest compensation for shrinkage by utilizing a parent glass body having dimensions slightly larger than the desired final product will suffice to satisfy the product needs. Where tight dimensional tolerances have been demanded, however, costly and time-consuming grinding or other machining techniques have been demanded. This shrinkage phenomenon has been especially worrisome in the recent practice of employing glass-ceramic materials in the preparation of dental restorations. As can well be appreciated, the fit of a dental construct is of utmost importance to the patient.
U.S. Application Ser. No. 373,617, filed Apr. 30, 1982 and entitled DENTAL PRODUCTS AND PROCESSES INVOLVING MICA COMPOSITIONS, now U.S. Pat. No. 4,431,420 discloses a process for the fabrication of dental tools, models, and constructs wherein the body thereof consists of a glass-ceramic having a composition within a narrowly-defined composition region to thereby yield material exhibiting the following six characteristics: (1) a visual appearance similar to that of tooth enamel; (2) inertness to chemicals encountered in an oral environment; (3) high mechanical strength and impact resistance to withstand the forces of mastication; (4) the capability of being processed via traditional laboratory techniques; (5) a coefficient of thermal expansion and a thermal conductivity similar to tooth enamel; and (6) the capability of being machined or otherwise mechanically shaped with relative ease, utilizing conventional metalworking tools, such as to permit the ready fashioning of the body to a desired anatomical configuration. Compositions providing that matrix of chemical and physical properties are reported as consisting essentially, expressed in terms of weight percent on the oxide basis, of
K.sub.2 O: 10-18 PA1 MgO: 14-19 PA1 SiO.sub.2 : 55-65 PA1 Al.sub.2 O.sub.3 : 0-2 PA1 ZrO.sub.2 : 0-7 PA1 F: 4-9 PA1 (1) a batch for a glass of a desired composition is melted; PA1 (2) the melt is simultaneously cooled and shaped to form a glass body having an intermediate configuration with at least one selected surface of a specified conformation; PA1 (3) the glass body is heat treated at about 1050.degree.-1150.degree. C. to cause the in situ growth of tetrasilicic fluormica crystals, thereby converting the glass body to a glass-ceramic body containing tetrasilicic fluormica as the predominant crystal phase; and thereafter PA1 (4) the glass-ceramic body is machined or otherwise formed to produce selected surfaces of the desired final geometry. PA1 (a) pressing a soft impression material against a dental surface to establish a shape and solidifying the shape to form an impression; PA1 (b) filling said impression with dental stone material and solidifying the dental stone material to from a model; PA1 (c) preparing a wax pattern from said model; PA1 (d) placing the pattern in an investment casting slurry on a sprue that extends from the pattern to a surface of the slurry and solidifying the slurry to form a mold; PA1 (e) removing the sprue and the pattern from the mold; PA1 (f) melting a glass preform of a desired composition; PA1 (g) heating the mold to an elevated temperature but below the melting temperature of the glass preform; and then PA1 (h) casting the melt into the mold to form a glass body having an intermediate configuration.
wherein SrO and BaO may optionally be substituted for up to 50% of the K.sub.2 O content on the molar basis. To secure the best chemical durability and resistance to food staining, the preferred compositions will contain 1-9% Al.sub.2 O.sub.3 +ZrO.sub.2, with the most preferred including at least 0.5% Al.sub.2 O.sub.3 and/or at least 2% ZrO.sub.2.
The inventive method disclosed contemplated four general steps:
As is observed in that disclosure, the shaping of the glass body having an intermediate configuration is carried out utilizing standard investment casting techniques. Hence, as is explained therein, the conventional investment casting process comprises:
In accordance with an illustrative working example, the glass body was removed from the mold and subsequently heat treated to convert it into the desired glass-ceramic body.
Further research has indicated that conducting the heat treating of glass preform outside the mold to effect crystallization in situ thereof results in such a high degree of distortion therein due to thermal slumping and shrinkage as to render the product virtually useless in many instances. Therefore, as a solution to the problem, crystallization of the glass preform was undertaken with the preform contained within a stable ceramic embedment. It was believed that the embodiment would prevent distortion due to thermal slumping and inhibit shrinkage. Unfortunately, the problem was not so simple that it could be readily solved with the commercially-available materials.
The goal of the dental laboratory is to produce finished castings that are about 5000-10,000 PPM (parts per million), equivalent to 0.5-1%, greater than the size of the die from which they were molded. This is accomplished by balancing the thermal expansions and shrinkages of the material being cast and the investment. The currently-marketed investment materials were formulated with metals in mind. However, the thermal contraction manifested by the metals employed in dental constructs is less than the sum of the thermal contraction and crystal densification which the glass-ceramic compositions disclosed in Ser. No. 373,617 undergo. That difference is sufficiently large to preclude the glass-ceramic castings from fitting on their respective dies.
One method for limiting the effect, but not the fact, of body shrinkage when the glass is crystallized in situ ("cerammed") to a glass-ceramic was attempted by filling the cavity of the casting with a commercially-available, rigid refractory material which impeded the inherent radial shrinkage of the casting. Densification of the casting still occurred and the concomitant stretching thereof resulted in a slight reduction in wall thickness (note FIG. 2), frequently accompanied with cracking and/or breaking. The investment employed, marketed by Whip-Mix Corporation, Louisville, Ky. under the name Hi-Heat Soldering Investment, consists of a mixture of quartz and plaster of Paris with added setting and shrinkage agents. Analysis of the fracture character indicated that the cracking took place as the crystallized body was cooled to room temperature after the ceramming treatment. This analysis is discussed below.
FIG. 1 illustrates that the precursor glass is at a temperature above its annealing point when it is subjected to the crystallization heat treatment. Accordingly, the viscosity of the glass at such temperature is sufficiently low that no significant stress can be built up. Therefore, the cracking and/or breakage encountered must occur during heating of the glass or cooling of the glass-ceramic.
As is indicated in FIG. 1, the densification (with concomitant shrinkage) experienced as the glass is converted into a glass-ceramic takes place in concert with the development of crystallization. Accordingly, the rate of densification is directly related to the rate of crystal development which, in turn, is a function of the heat treating temperature. As is manifest in FIG. 3, the breakage almost always involves a crack running from the margin to the top of the casting, customarily along one side. The crack is closed, thereby implying that the casting did not go through a plastic (softening) stage after cracking. Furthermore, the fracture surface does not exhibit the skin which commonly develops on the exposed surfaces of the castings during the crystallization treatment. Consequently, the crack was self-evidently produced during cooling of the glass-ceramic.
As observed, the commercial investment material utilized consisted substantially of quartz and plaster of Paris (CaSO.sub.4.1/2H.sub.2 O). When mixed with water, the plaster is hydrated to monoclinic gypsum (CaSO.sub.4.2H.sub.2 O). When fired, water is driven off and water-soluble hexagonal anhydrite (CaSO.sub.4) is formed. At a temperature in the vicinity of 350.degree. C., the hexagonal CaSO.sub.4 is transformed into a slightly water-souble orthorhombic form. This transformation is accompanied with a density change of about 13%, which corresponds to a linear shrinkage of approximately 4.3%. Because of that intrinsic shrinkage of CaSO.sub.4, quartz is added thereto to compensate therefor. Quartz is subject to an inversion at 573.degree. C. which is accompanied by a large increase in volume. The resultant phase, termed .beta.-quartz, exhibits a coefficient of thermal expansion of essentially zero. FIG. 4 represents what are believed to be the thermal expansion characteristics of the commercial embedment material during the crystallization heat treatment (Curve I), along with the thermal expansion characteristics of a typical glass-ceramic of Ser. No. 373,617 (Curve II).
With the glass compositions of Ser. No. 373,617, the annealing points thereof range between about 600.degree.-625.degree. C., the onset of crystallization occurs at about 650.degree.-700.degree. C., and the completion of the desired tetrasilicic fluormica crystallization takes place at about 1050.degree. C. FIG. 4 demonstrates that the commercial investment shrinks somewhat at about 1000.degree. C., i.e., below the temperature utilized for crystal growth of the glass compositions, this shrinkage resulting from sintering and consolidation of the material. Accordingly, if the annealing points are deemed to effectively represent the setting points of the glasses, then, when the cavity of a crown construct casting is completely filled with an embedment material, the critical inside dimension of the construct will conform to the geometry of the embedment as the construct is heated to effect crystallization thereof. Upon cooling from the crystallization temperature region, the glass-ceramic construct will continue to accommodate itself to the embedment until the setting point of the glass-ceramic (about 775.degree.-825.degree. C.) is reached and the casting can begin to support stress.
Unfortunately, because of the wide disparity existing between the coefficients of thermal expansion of the embedment material and the glass-ceramics of Ser. No. 373,617, the casting is placed in tension (the shaded portions of FIG. 4 indicate when the casting is in tension) and cracking of the construct occurs during cooling over the range of temperatures between about 800.degree.-500.degree. C.
Therefore, the principal objective of the instant invention is to develop materials operable as embedments for heat treating glass castings, whereby the shrinkage customarily experienced resulting from densification of the body as the glass is converted into a glass-ceramic will be minimized and glass-ceramic articles of tight dimensional tolerances can be produced.
The conventional method employed in dental laboratories to compensate for shrinkage inherent in the making of alloy restorations, viz., adjusting the investment liquid:power ratio, is not applicable with glass-ceramic materials. That is, the magnitude of the size change that can be effected by that technique is not adequate with the compositions disclosed in Ser. No. 373,617. Accordingly, a specific objective of the present invention is to devise an embedment material which will permit the preparation of glass-ceramic dental constructs, models, and tools which are free from cracking and/or breakage via the heat treatment of glass castings having compositions disclosed in Ser. No. 373,617.